Timestamp: `set_timestamp` sets `DidUpdate` (#11960)

* Timestamp: `set_timestamp` sets `DidUpdate`

There exists the `set_timestamp` in the Timestamp pallet for setting the current timestamp. The
problem is that it doesn't set `DidUpdate`. This results in `on_finalize` panicking. There is no
real reason why the function doesn't also set `DidUpdate`.

* Update frame/timestamp/src/lib.rs

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Fix Babe tests

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>